An electrophotographic printer forms an electrostatic latent image on a photosensitive drum by light according to an image signal, develops the latent image by selectively attracting toner thereto, and then transfers the developed image onto a paper to obtain a print image. As a light source for forming the electrostatic latent image, a laser and a light-emitting diode array are widely used. In particular, since a light source constituted by the light-emitting diode array does not need a long optical path unlike the laser-type light source, it is suitable for small-sized printers. Because the light-emitting diode array can be laterally long, it is also suitable for large-sized printing. Demand has recently been mounting on light-emitting diode arrays of higher precision, higher light output and lower in cost, as printing has been becoming faster with higher image quality, and as printers have been becoming smaller.
To achieve the reduction of the cost of light-emitting diode array heads, it is advantageous to replace a static driving system comprising ICs for separately driving light-emitting diodes, with a dynamic driving system or a matrix driving system, which comprises pluralities of light-emitting diodes in a block, and a switching matrix wiring subjected to time division operation, thereby reducing the number of driving ICs and bonding wires (see “Oki Technical Review,” Vol. 69, No. 1, January, 2002, Ser. No. 189).
In view of the above circumstances, the inventors have proposed a light-emitting diode array of a partitioned-matrix, dynamic driving system with the number of bonding pads reduced by connecting first bonding pads to first electrodes via common wirings such that the ratio of the number of the first bonding pads connected to the first electrodes to the number of the second bonding pads connected to the second electrodes is 1:n (n 3) (Japanese patent application No. 2003-289909 corresponding to U.S. Ser. No. 10/910,658). FIG. 7 is a top view showing a light-emitting diode array of a four-partitioned matrix driving system having such a structure. In this example, one cell is composed of 64 dots of light-emitting portions. Bonding pads 6c, which are connected to first electrodes (cathodes) 2 via common switching wirings 4, and bonding pads 6a, which are connected to second electrodes (anodes) 3, are arranged in such a row that the ratio of the number of the bonding pads 6c to the number of the bonding pads 6a is 1:4.
FIG. 8 is an enlarged top view showing part of the light-emitting diode array shown in FIG. 7. FIG. 9(a) is a cross-sectional view taken along the line C—C in FIG. 8, FIG. 9(b) is a cross-sectional view taken along the line D—D in FIG. 8, and FIG. 9(c) is a cross-sectional view taken along the line E—E in FIG. 8. This light-emitting diode array comprises a substrate 10, a conductive layer 11 formed on the substrate 10, pluralities of light-emitting portions 1 formed on the conductive layer 11, cathodes 2 each formed on an upper surface of each light-emitting portion 1, anodes 3 formed on the conductive layer 11, four common wirings 4 separately connected to the cathodes 2 via lead wires 5c, bonding pads 6c separately connected to the common wirings 4, and bonding pads 6a separately connected to anodes 3 via lead wires 5a. As shown in FIG. 8, the light-emitting portions 1 are partitioned by a first rectangular groove 20 formed in the conductive layer 11 to blocks such that each block comprises four light-emitting portions 1.
FIG. 10 is a top view showing a straight current 26a as a driving current and a sneak current 26b both flowing to the light-emitting portions 1 in one block of this light-emitting diode array. FIG. 11 is a top view showing the light-emitting portion 1 and its surroundings. The cathode 2 formed on the upper surface of the light-emitting portion 1 is in a T shape comprising a connection portion 2a extending from an end (on the farther side from the anode 3) substantially in the width of the light-emitting portion 1, and an elongated portion 2b extending from the connection portion 2a on a center portion of the light-extracting portion 9. Accordingly, not only a straight current 26a flowing from the anode 3 immediately below the light-emitting portion 1 but also a sneak current 26b can be guided to a region immediately below the light-extracting portion 9.
Light output can be improved by adjusting the width of the light-extracting portion 9 and the length of the cathode 2. FIGS. 12(a) and 12(b) show the relation between the length L of the connection portion 2a of the cathode 2, and light output and driving voltage, respectively. As shown in FIGS. 12(a) and 12(b), by setting the length L of the connection portion 2a to 20 μm or less, the light output can be drastically improved while suppressing a driving voltage. The extension portion 2b of the cathode 2 may have a shape comprising pluralities of stripes, networks, etc., as long as fine working is possible.
The second grooves 121 formed on the conductive layer 11 in each block substantially extend from a side (on the farthest side from the anode 3) of the first groove 20 to ends (on the farthest side from the anode 3) of the light-extracting portions 9 between the adjacent light-emitting portions 1. The second grooves 121 suppress those having no contribution to light emission among the sneak current, so that those having contribution to light emission (sneak current 26b) can flow immediately below the light-extracting portion 9 at a high efficiency. This increases the light output, enabling low-voltage driving.
However, the sneak current 26b flows from one side to the light-emitting portion 1a adjacent to the first groove 20, while the sneak current 26b flows from both sides to the light-emitting portion 1b, which is not adjacent to the first groove 20. Accordingly, the amounts of current flowing to the light-emitting portions 1a and 1b are different from each other.
FIG. 13(a) shows an example of the light output of each light-emitting portion 1 in a block, and FIG. 13(b) shows an example of the driving voltage of each light-emitting portion 1 in a block. As is clear from FIGS. 13(a) and 13(b), the light output and driving voltage of light-emitting portions 1 are relatively uneven in each block.